1. The Field of the Invention
The present invention relates to systems and methods for modulating lasers. More particularly, the present invention relates to systems and methods for adjusting or compensating a modulation current of a laser according to temperature.
2. The Relevant Technology
Optical networks have the ability to transmit data using light or optical signals at high transmission speeds. The optical signals are usually generated by an optical transmitter, which converts an electrical signal into an optical signal. Optical transmitters often use a laser diode to generate the optical signal, although some applications use light-emitting diodes.
In order to convey data in an optical network, the data needs to be incorporated into the optical signal generated by the laser and then launched in an optical fiber. The data is encoded in the signal by modulating the laser current according to the data that is being transmitted. Perhaps the simplest way to modulate a laser is to turn the laser on for a data bit that is a 1 and turn the laser off for a data bit that is a 0. Thus, the data bits can be distinguished based on their optical power.
Experience has shown, however, that the turn off/on times of a laser can affect the transmission speed of the laser. As a result, the laser may not be turned completely off during the transmission of a data bit that is a 0. Therefore, a relatively high optical power indicates a 1 and a relatively low optical power corresponds to a 0. Data bits are still distinguished by their relative optical power and their relationship may be described by the extinction ratio.
The ability of a laser to successfully transmit an optical signal can depend on many factors, one of which is temperature. For example, the threshold current or the current at which a laser begins to lase is dependent on temperature. FIG. 1 illustrates this aspect of a laser and depicts a plot of the threshold current of a laser with respect to temperature. In FIG. 1, the curve 101 illustrates that as temperature changes, the threshold current of the laser also changes. The lowest threshold current appears at point 104, which is associated with a certain temperature T2. As the temperature increases to the temperature T3, the point 106 shows that the threshold current is higher than the threshold current at temperature T2. Similarly, as the temperature decreases to T1, the point 102 illustrates an increased threshold current with respect to the threshold current at temperature T2 and the point 104.
FIG. 2 further illustrates one of the effects of temperature on lasers. FIG. 2 shows that at different temperatures, the modulation current needed to modulate the laser also changes. More specifically, FIG. 2 illustrates a graph of optical power with respect to the modulation current and temperature. The curve 214, for example, illustrates the average power 202 for the laser at a first temperature and the modulation 206 and 208 associated with the curve 214. In contrast, the curve 216 illustrates the modulation 210 and 212 required for the same or similar laser at a different temperature. The slope of the curve 216 is flatter than the slope of the curve 214, which indicates that more modulation current is required at higher (or lower) temperatures to achieve similar optical power at the laser output.
If the modulation current used in a laser is not compensated for changes in temperature, then the output power of the laser will also change. This may be reflected in the extinction ratio of the laser. For example, the extinction ratio of the optical signal may decrease as temperature of the laser increases. It is therefore desirable to adjust the modulation current of the laser to compensate for changes in temperature. If the modulation current were adjusted to compensate for temperature changes, the extinction ratio is more constant and the optical signal can be more easily received by an optical receiver.
The problem of adjusting the modulation current has typically been approached using a look up table that is based on the temperature. A look up table, however, cannot continuously adjust the modulation current in response to changes in temperature. A look up table further cannot be adjusted to account for the differences that are often present in different lasers. For example, the temperature coefficient of one laser may be quite different from the temperature coefficient of another laser. As a result, the look up table may not provide the proper information for a particular laser. In short, a look up table may not provide the flexibility that is required to continuously adjust the modulation current of a laser.